Abstract: ARCON96 model is the latest NRC recommended model for design basis of control room habitability assessments at nuclear power plants, which was developed with the improvement in atmosphere relative concentration estimation for calm wind and short distance. The computation behavior of ARCON96 model was studied and was compared with Gaussian model in this paper. The ARCON96 model estimated atmosphere relative concentration is a function of both the wind speed and distance. For short distances, the relationship between atmosphere relative concentration and wind speed can be described approximately using a bell-shaped curve. As the distance increases, the bell-shaped curve gradually degenerates into an exponential decay curve. Meanwhile, the meteorological condition that corresponds to the maximum atmosphere relative concentration changes from non-calm wind to calm wind. At all distances, the atmosphere concentration estimated by ARCON96 model is smaller than that of Gaussian model.